What is a bioretention island:
Bioretention islands are landscaping features adapted to treat stormwater
runoff. They are most frequently located within parking lot islands but
can also be incorporated into cul-de-sacs and small pocket gardens in
residential land uses. Instead of raised planters, bioretention islands
are depressions that collect and filter the first ½ inch of rainfall
off paved surfaces. Bioretention islands are designed so that surface
runoff is directed into the vegetated islands. The vegetation
absorbs stormwater and filters pollutants from the runoff. Often, the
filtered runoff is collected in a perforated pipe under the island and
returned to the storm sewer system. In larger storm events, runoff is
diverted past the island to a storm drain.

Bioretention islands
are typically used for stormwater management in small drainage areas,
such as a small parking lot, or an individual residential property. However,
they are quite versatile in that they can be employed in almost any soil
condition. Even existing parking lot landscape islands can be retrofitted
to incorporate bioretention.

To enhance pollutant
removal, the bioretention island should be sized to be between 5% and
10% of the impervious
area draining to it. The underlying planting bed should be designed as
a sand/soil mix with a mulch layer above the soil. The surface of a bioretention
area is usually planned so that it ponds a small depth of water (6-9 inches)
above the filter bed. And some bioretention islands also are designed
to help spread flows evenly and settle out large particles.

Why
consider bioretention islands:Bioretention
islands remove of a wide variety of pollutants, including suspended solids
and nutrients. They can be employed to improve water quality in areas
that often generate a variety of pollutants (parking lots, roadways).
Most municipalities already require landscaping islands in land development
proposals, so some very simple plan modifications could go a long way
towards addressing existing stormwater management requirements and increasing
water quality at the same time.

Limitations:
There are very few limitations to bioretention island applications besides
their requirement to treat small drainage areas.

Benefits:
By now, most of us realize that traditional storm water management systems
empty runoff into local streams without any treatment. The main benefit
of bioretention islands is their pollutant removal capability. Bioretention
islands are easy to fit into a project in urban areas where land availability
for traditional facilities is scarce.

Maintenance:
Monthly inspections are recommended until the plants are established.
Afterwards, seasonal landscaping maintenance is required for bioretention
islands, similar to those required for all planting beds (remulching,
treating diseased shrubs and trees, watering, removing litter and debris).
Bioretention islands should be designed so that they are easily accessible
for maintenance.

Materials:
Plant selection is critical to the function and appearance of the bioretention
islands. Native vegetation should be incorporated as much as possible.
Ideal plants include those that can tolerate both wet and dry conditions
and tolerate pollutants associated with stormwater runoff.